The present invention relates to a plasma treatment apparatus for subjecting an object to be treated, for example, a semiconductor wafer to a treatment such as film etching or film formation.
A plasma treatment is widely used in manufacturing processes for semiconductor wafers due to its high accuracy in process control. Plasma treatment apparatuses include a wafer-by-wafer treatment type and a batch type. A wafer-by-wafer etching apparatus, by way of example, comprises a vacuum treatment chamber, and upper and lower electrodes vertically confronting each other within the vacuum chamber. A semiconductor wafer is mounted on the lower electrode, and a high-frequency or RF (radio frequency) electric power is supplied between the two electrodes. In such a plasma treatment the wafer is required to be uniformity maintained at a predetermined temperature, and the apparatus therefore has, on the side of the lower electrode, temperature control means and means for supplying a heat transfer backside gas.
The known plasma treatment apparatus further comprises a grounded member which is provided below the lower electrode and is continuous with a wall of the vacuum treatment chamber by way of an electrically insulating member. The lower electrode is electrically connected to a RF power source while the grounded member is connected to the earth. The upper electrode is electrically connected to the grounded member, whereby an RF electric power can be applied between the upper and lower electrodes.
A gas supply conduit for a backside gas, made of an electrically insulating material is provided so as to extend from below the grounded member, that is, from below the vacuum treatment chamber, through the grounded member and electrically insulating member, to the underside of the lower electrode. The top end of the gas supply conduit communicates, via an accumulator passage and an accumulator space provided within the lower electrode, with a multiplicity of gas emission holes. Within the lower electrode there is also provided a cooling medium reservoir allowing passage of the cooling medium therethrough.
At the time of etching treatment, the wafer is attracted onto the lower electrode with the aid of an electrostatic chuck, and a backside gas, for example, He gas from the gas supply conduit is blown onto the backside of the wafer through the gas emission holes, thereby providing a uniform distribution of temperature over the wafer surface. After the completion of the etching, the electrostatic chuck is turned off, while simultaneously sucking the backslide gas through the gas supply conduit so as to prevent the wafer from being blown off by the pressure of the backside gas remaining within the gas supply conduit.
In the above-described etching apparatus, however, there was a fear of an electric discharge occurring between the lower electrode and the grounded member by way of the gas supply conduit for a reason which will be described later. The occurrence of such electric discharge makes it impossible to secure a predetermined electric power energy, which will lead to a reduction in the etching rate. Thus, unawareness of an electric discharge will result in an insufficient etching treatment. Further, unstabilized plasma may prevent a matching of the impedance, and due to the electric discharge a damage to the parts such as the gas supply conduit and electrically conducting sections will occur. It is to be appreciated that if the gas supply conduit is of small diameter to lower the voltage at which electric discharge occurs, the conductance will become small and it will take a considerable amount of time to suck the backside gas, thus resulting in a reduced throughput.
In this type of plasma treatment apparatus, the electrostatic chuck for holding an object to be treated such as a semicnductor wafer is provided with an electrostatic attraction sheet having a multiplicity of openings through which a backside gas, for example, He gas is supplied and filled between the object to be treated and the electrostatic attraction sheet. Such filling of the gas ensures a uniform heat transfer between the object to be treated and the electrostatic attraction sheet. This type of plasma treatment apparatus has the problems stated below, in particular, when a mount supporting the electrostatic chuck is cooled to restore the temperature within the treatment chamber to the room temperature from an ultra lower temperature at which the object to be treated undergoes a plasma treatment.
Within a tank as the source of supply of the backside gas, moisture may be mixed into the backside gas such as He gas to be filled between the object to be treated and the electrostatic sheet. This moisture is caused to return to the liquid phase as the temperature approaches the room temperature, the moisture having been condensed in the gas accumulation sparse and gas passage during the plasma treatment under low temperature conditions. As long as this liquid phase water remains on the inner wall surface of the gas accumulation space, there arises no problem. However, there is a large possibility for the liquid phase water to be emitted onto the attraction surface between the electrostatic chuck and the object to be treated. Thus, in the case of the liquid phase water depositing on the attraction surface, there occurs a residual electric charge on the surface of the electrostatic chuck under the influence of the presence of hydroxyl groups (OH.sup.-, OH.sup.+) involved in the water. The occurrence of the residual electric charge on the electrostatic chuck will naturally induce a residual electric charge on the object to be treated immediately confronting and abutting against the electrostatic chuck.
It is therefore necessary when unloading the object to be treated that any such residual electric charge and any electric charge remaining after the induction by the application of voltage to the electrostatic chuck be eliminated to assist in unloading the object to be treated. To this end, the elimination of the electric charge has hitherto been effected when pushing up the object to be treated by pusher pins which are commonly used in this type of apparatus and which serve as grounded members.
In case there exists a large amount of residual charge, however, the above constitution will result in an increase in the number of times by which the object to be treated is pushed up by the pins. Accordingly, it takes a considerable time to complete the unloading of the object to be treated, resulting in a poor throughput. Apart from this, there is a fear of damaging or impairing the surface of the object to be treated with increased number of times of the push up. Furthermore, the presence of the hydroxyl groups will induce a deposition of an unnecessary oxide film on the surface of the semiconductor wafer, which brings about unfavorable results in view of the characteristics of the semiconductor wafer.
In the case of filling a gas such as He gas, a uniform heat transfer can be accomplished between the electrostatic chuck and the object to be treated, whereas such gas may possibly leak from the filling space to the exterior. Should leakage occur, liquid phase water which is produced upon the return to the room temperature and contained in the gas will be scattered within the plasma treatment chamber or will adhere to the wall surface thereof.
Thus, when evacuating the interior of the plasma treatment chamber, removal must be performed of the filling gas which has leaked out as well as the liquid phase water which has been produced with the return to the room temperature, which inevitably elongates the time taken for the evacuation. An incomplete removal of the water will adversely affect the conditions of the plasma treatment to be subsequently executed.